1. Field of the Invention
The present invention relates to a thermal transfer sheet for use in an imaging process wherein a high-resolution image is formed by using a laser light. More specifically, the present invention relates to a thermal transfer sheet that can be used in an imaging process for preparation of a color proof (DDCP: direct digital color proof) or of a mask image, in printing with laser recording from digital image signals.
2. Description of the Related Art
In the field of graphic art, an image is printed on a printing plate by using a set of color-separation films prepared from a color original by using lithographic films. And, usually, a color proof is prepared from the color-separation films in order to check for any errors in a color separation process or for the necessity of color correction before a final printing (i.e., actual printing) process. The color proof is desired to have the capability of actualizing high resolution which enables a high-fidelity reproduction of halftone images, as well as a high level of process stability, and the like. In addition, in order to obtain a color proof approximating actual printed matter, the materials for the color proof are desirably the same materials as those for the actual printed matter. For example, the substrate of the color proof is desired to be the printing paper for use in the printed matter and the coloring materials of the color proof are desired to be pigments. Further, it is highly desired to adopt a dry process, which does not use a liquid developer, as a method for the preparation of the color proof.
As for the dry-process preparation method of the color proof, a recording system, capable of preparing the color proof directly from digital signals, has been developed as a result of the wide spread use of an electronically controlled system in recent pre-printing processes (pre-press field). In such an electronically-controlled system, the preparation of a color proof of a particularly high quality is necessary, and therefore the reproduction of halftone dots having 150 lines per inch or more is generally required. And, in order to record a high-quality proof from digital signals, it is necessary to use, as a recording head, a laser light that can be modulated by the digital signals and can be focused to a small concentrated spot. Accordingly, there is a demand for a recording material that exhibits a high recording sensitivity with respect to the laser light and a high resolution enabling the reproduction of highly precise and fine halftone dots.
As for a recording material for an imaging process by transfer using a laser light, hitherto known is a heat-fusion type transfer sheet (described in Japanese Patent Application Laid-Open (JP-A) No. 5-58,045) comprising a substrate, a light-to-heat conversion layer which generates heat by absorbing the laser light, and an image forming layer which has a pigment dispersed in a component such as a heat-sensitive fusible wax, a binder or the like, disposed in that order on the substrate. According to this imaging process using these recording materials, heat generated in a region irradiated with the laser light in the light-to-heat conversion layer fuses the image forming layer correspondingly to the irradiated region so that the fused layer is transferred to an image receiving sheet disposed on the transfer sheet. In this way, a transferred image is formed on the image receiving sheet.
In addition, JP-A No. 6-219,052 discloses an imaging process using a thermal transfer sheet comprising a substrate, a light-to-heat conversion layer containing a substance capable of converting light to heat, a very thin (0.03.about.0.3 .mu.m) heat-sensitive peelable layer, and an image forming layer containing a coloring material, disposed in that order on the substrate, wherein, upon irradiation with a laser light, the interposed heat-sensitive peelable layer diminishes the adhesion between the image forming layer and the light-to-heat conversion layer. In this way, a highly precise, fine image is formed on the image receiving sheet disposed on the thermal transfer sheet. This imaging process utilizes a so-called "abrasion". More specifically, in the region irradiated with the laser light, part of the heat-sensitive peelable layer is decomposed and vaporized. As a result, the adhesion between the image forming layer and the light-to-heat conversion layer decreases so that the image forming layer in the irradiated region is transferred to the image receiving sheet disposed on the thermal transfer sheet.
The above-mentioned imaging processes are advantageous in that a printing paper having formed thereon a receptor layer (bonding layer) can be used as a material for the image receiving sheet and in that a multicolor image can be easily obtained by sequentially transferring images of different colors to the image receiving sheet. In particular, the imaging process using abrasion is advantageous in that a highly precise, fine image can be easily obtained and is therefore useful for the preparation of a color proof (DDCP :direct digital color proof) or a highly precise, fine mask image.
The layers of the thermal transfer sheet for use in the above-mentioned imaging processes are formed by sequentially coating the layers on a substrate. Therefore, the layers need to be formed easily. In addition, the binder for the formation of the light-to-heat conversion layer, which comprises a substance capable of converting light to heat (usually a dye capable of absorbing laser light) and a binder, needs to have, for example, a capability of easily dispersing therein the substance capable of converting light to heat and excellent heat resistance. Traditionally, examples of the binder for the light-to-heat conversion layer include homopolymers or copolymers of acrylic monomers such as acrylic acid; cellulosic polymers such as cellulose acetate; vinyl-based polymers such as polystyrene, vinyl chloride/vinyl acetate copolymers, polyvinyl butyral, and polyvinyl alcohol; polymers made by polycondensation such as polyesters and polyamides; rubber-based thermoplastic polymers such as butadiene/styrene copolymers; polyurethanes; epoxy resins; and urea/melamine resins, as described in, e.g., JP-A Nos. 5-58,045and 6-219,052. Among these polymers, polymers such as polyvinyl alcohol, polyvinyl butyral, and polyesters, are ordinarily preferable.
According to studies conducted by the present inventors, however, disadvantages of the prior art are as follows. The light-to-heat conversion layer, which uses a water-soluble polymer such as polyvinyl alcohol, is generally inferior in humidity resistance and therefore sometimes caused dyes to agglomerate during storage for a long period of time under conditions of high temperature and high humidity. On the other hand, if a resin, which is less liable to cause the above-mentioned phenomenon and which is exemplified by polyvinyl butyral, a polyester resin or the like, was used for forming of the light-to-heat conversion layer, the light-to-heat conversion layer was moistened by a solvent contained in a coating liquid for forming a heat-sensitive peelable layer, to be coated on the light-to-heat conversion layer, or alternatively by a solvent contained in a coating liquid for forming an image forming layer, to be coated on the light-to-heat conversion layer. As a result, the dye in the light-to-heat conversion layer was transferred to these other layers, and sometimes the performance (e.g., sensitivity) of the light-to-heat conversion layer was worsened or fogging occurred. In addition to these disadvantages, since the heat resistance of these polymers was insufficient, these polymers were liable to cause thermal decomposition and adhesion due to fusion. As a result, a portion of the light-to-heat conversion layer was transferred together with the image forming layer, which sometimes inhibited preparation of a satisfactory image or presented an impediment to a transfer operation.
Accordingly, an object of the present invention is to provide a thermal transfer sheet whose light-to-heat conversion layer is not affected by the coating liquids for forming layers thereon and has a high level of heat resistance and humidity resistance, and which produces a good transferred image with minimal fogging.